1. Field of Invention
This invention relates generally to electrical interconnection systems and more specifically to improved signal integrity in interconnection systems, particularly in high speed electrical connectors.
2. Discussion of Related Art
Electrical connectors are used in many electronic systems. It is generally easier and more cost effective to manufacture a system on several printed circuit boards (“PCBs”) that are connected to one another by electrical connectors than to manufacture a system as a single assembly. A traditional arrangement for interconnecting several PCBs is to have one PCB serve as a backplane. Other PCBs, which are called daughter boards or daughter cards, are then connected to the backplane by electrical connectors.
Electronic systems have generally become smaller, faster and functionally more complex. These changes mean that the number of circuits in a given area of an electronic system, along with the frequencies at which the circuits operate, have increased. Electrical connectors are needed that are electrically capable of handling more data at higher speeds. As signal frequencies increase, there is a greater possibility of electrical noise being generated in the connector, such as reflections, crosstalk and electromagnetic radiation. Therefore, the electrical connectors are designed to limit crosstalk between different signal paths and to control the characteristic impedance of each signal path.
Shield members can be placed adjacent the signal conductors for this purpose. Crosstalk between different signal paths through a connector can also be limited by arranging the various signal paths so that they are spaced further from each other and nearer to a shield, such as a grounded plate. In this way, the different signal paths tend to electromagnetically couple more to the shield and less with each other. For a given level of crosstalk, the signal paths can be placed closer together when sufficient electromagnetic coupling to the ground conductors is maintained. Shields for isolating conductors from one another are typically made from metal components. U.S. Pat. No. 6,709,294 (the '294 patent) describes making an extension of a shield plate in a connector made from a conductive plastic.
Other techniques may be used to control the performance of a connector. Transmitting signals differentially can also reduce crosstalk. Differential signals are carried on by a pair of conducting paths, called a “differential pair.” The voltage difference between the conductive paths represents the signal. In general, a differential pair is designed with preferential coupling between the conducting paths of the pair. For example, the two conducting paths of a differential pair may be arranged to run closer to each other than to adjacent signal paths in the connector. No shielding is desired between the conducting paths of the pair, but shielding may be used between differential pairs. Electrical connectors can be designed for differential signals as well as for single-ended signals. Examples of differential electrical connectors are shown in U.S. Pat. No. 6,293,827. U.S. Pat. No. 6,503,103, U.S. Pat. No. 6,776,659, U.S. Pat. No. 7,163,421, and U.S. Pat. No. 7,581,990.
Electrical characteristics of a connector may also be controlled through the use of absorptive material. U.S. Pat. No. 6,786,771 describes the use of absorptive material to reduce unwanted resonances and improve connector performance, particularly at high speeds (for example, signal frequencies of 1 GHz or greater, particularly above 3 GHz). And, U.S. Pat. No. 7,371,117 describes the use of lossy material to improve connector performance. These patents are all hereby incorporated by reference.